Liquid refrigerant feed control

ABSTRACT

The quality of the suction gas at the input of a variable capacity compressor in a refrigeration system is controlled by regulating the flow of liquid refrigerant from the condenser to the evaporator in the system. A thermistor at the input of the compressor senses the quality or gas-liquid ratio of the suction gas and controls the position of a liquid refrigerant flow valve to control the flow rate of the liquid refrigerant, thereby controlling the quality of the suction gas at the input to the compressor. The position of the liquid refrigerant flow valve may also be controlled in accordance with the capacity setting of a variable capacity compressor.

llite States Patent [191 l 1 amilton [451 May 29,1975

[541 LIQUID REFRIGERANT FEED 2,583,178 1/1952 Huntington ..e2/223CONTROL 3,397,552 8/1968 l-larnon ..62/202 Inventor: Clark B. Hamilton,Hartford, Conn.

Dunham-Busli, Hartford,

Conn.

Filed: Aug. 12, 1971 Appl. No.: 171,081

Assignee: Inc.,

References Cited UNITED STATES PATENTS Matthies ..62/225 ....62/225....62/ 196 ..62/223 Long Soumerai Koontz Primary Examiner-Meyer PerlinA tt0rney- Richard C. Sughrue, Gideon Franklin Rothwell,.lohn H. Mioneta1.

[57] ABSTRACT The quality of the suction gas at the input of a variablecapacity compressor in a refrigeration system is controlled byregulating the flow of liquid refrigerant from the condenser to theevaporator in the system. A thermistor at the input of the compressorsenses the quality or gas-liquid ratio of the suction gas and controlsthe position of a liquid refrigerant flow valve to control the flow rateof the liquid refrigerant, thereby controlling the quality of thesuction gas at the input to the compressor. The position of the liquidrefrigerant flow valve may also be controlled in accordance with thecapacity setting of a variable capacity compressor.

- PAIENIE w $735,803

POWER INPUT I LIQUID REGERANT FEED CONTROL BACKGROUND OF THEINVENTION 1. Field of the Invention The invention relates generally to arefrigeration system including a compressor, condenser, and evaporator,and more particularly to a means and method of controlling the flow rateof liquid refrigerant through the evaporator.

2. Description of the Prior Art In the prior art, in a refrigerationsystem including a flooded chiller-evaporator, a condenser, and avariable capacity compressor having a variable capacity capability, suchas disclosed in US. Pat. No. 3,408,827, the compressor load or capacityis determined by a control which is responsive to a sensor which sensesthe temperature of the cooling water leaving or entering the evaporator.The water is maintained at a constant tem perature by activating adevice in the compressor which varies the compessor capacity, i.e. thecompression ratio, or more specifically, the pumped mass flow rate. Thisvarying rate of mass flow through the compressor, and consequentlythrough the condenser and evaporator, requires a variable liquid controldevice for modulating the liquid refrigerant mass flow from thecondenser to the evaporator more or less in response to the compressormass flow rate, to prevent over or under-feeding of the evaporator andthe resulting system shutdown.

This pprevention of overor under-feeding is normally done by one of thefollowing means:

l. Maintaining a constant liquid level in the condenser (i.e. high sidefloat);

2. Maintaining a constant liquid level in the evaporator (i.e. low sidefloat);

3. Variable subcooling control (i.e. fixed orifices in series 4. Aconstant evaporator pressure regulator; and

5. A thermal expansion valve control.

All of these foregoing methods have various drawbacks in that somecompromise has to be made in their application with a resulting adverseeffect on system performance, cost or operation range.

SUMMARY OF THE INVENTION The board object of the invention is to providean improved method and means of liquid refrigerant feed control in arefrigeration system.

A more specific object of the invention is to provide an improved methodand means of maintaining the compressor mass flow rate and the liquidrefrigerant mass flow rate from the condenser to the evaporator in therefrigeration system substantially equal.

Another object of the invention is to sense the quality of the suctiongas at the input of the compressor and to control the liquid refrigerantmass flow rate from the condenser to the evaporator in accordance withthe sensed quality, in order to keep the suction gas quality at adesired optimum value.

Still another object of the invention is to control the refrigerant massflow rate from the chiller to the condenser in accordance with the loador capacity of a variable capacity compressor, thereby maintaining thecompressor mass flow rate and the liquid flow rate from the condenser tothe evaporator at substantially the same value without time lag.

A more specific object of the invention is to control the position of aliquid refrigerant control valve between the condenser and theevaporator by the output of an electrical bridge circuit which hasinputs corresponding both to the compressor capacity setting and to thequality of the suction gas at the input to the compressor.

The invention may be briefly summarized as a liquid refrigerant feedcontrol system for a variable capacity compressor in a refrigerationsystem wherein the position of a liquid refrigerant flow control valveplaced between the condenser and the evaporator is controlled directlyby the compressor capacity setting to maintain the liquid refrigerantflow rate into the evaporator substantially equal to the compressor massflow rate. F urthermore, a refrigerant vapor quality sensing means maybe placed in the input line of the compressor to sense the quality ofthe compressor suction gas to make fine adjustments in the position ofthe flow control valve to maintain the suction gas quality at a desiredoptimum value regardless of changes in the system refrigerant charge,operating pressures, oil foaming in the evaporator, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of oneform of refrigeration system to which the liquid refrigerant control ofthis invention may be applied.

FIG. 2 is a schematic diagram of a preferred embodiment of the liquidrefrigeratnt feed control of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates aconventional prior art refrigeration system in which the liquidrefrigerant feed control of this invention is particularly useful. Avariable capacity, i.e. variable displacement compressor 10 pumpsrefrigerant at a certain mass flow rate through a condenser 12 and anevaporator or chiller 14 and then back to the input or suction line 16of the compressor. A liquid refrigerant flow control valve 20 isinserted in the line between the condenser and the evaporator. In theconventional operation of this type of system, the compressor iscontinuously operating and the cooling capacity or displacement of thecompressor is determined by the position of a capacity control slidevalve 22 whose position is controlled by a temperature controller TCwhich in turn is controlled by a thermostat TS which senses thetemperature of the water entering or leaving the evaporator-chiller 14.Conventionally, the compressor is continuously running, and the positionof the valve 20 is controlled by one of the means outlined above.

FIG. 2 illustrates a preferred embodiment of the present invention asapplied to the refrigeration system illustrated in FIG. 1. The samereference numerals have been used to identify corresponding componentsof FIGS. 1 and 2.

The preferred embodiment of the liquid refrigerant feed control of thepresent invention consists of a bridge circuit 24 having a pair of inputterminals 26 and 28 across which may be connected a suitable source ofac. or dc. power. The bridge circuit 24 consists essentially of fourvariable resisistance devices: a rheostat 28 having a movable wiper arm30 which is adjusted to calibrate the circuit; a potentiometer 32 havingan adjustable wiper arm 34 which is connected by a mechanical linkage 36to the compressor capacity control slide valve 22 in the variablecapacity compressor a selfheated thermistor 38 which is placed insidethe compressor input or suction line 16 to sense the quality of thesuction refrigerant gas; and a potentiometer 40 having a movable wiperarm 42 which is connected via a mechanical linkage 44 to a reversiblemotor 46 which controls the position of valve 20 and thereby the liquidrefrigerant flow rate between condenser 12 and the evaporator 14.Therefore, the position of wiper arm 42 on the potentiometer 40 iscontinuously determined by the position of the valve 20.

When a voltage is applied across the terminals 26 and 28, electricalcurrent flows from terminal 26 through two parallel paths in the bridgecircuit 24 back to terminal 28. The first path is from input terminal 26through the wiper arm 34 and the left-hand portion of potentiometer 32through the rheostat 28 and the wiper arm 30, and then through a relaycoil 50, the lefthand portion of potentiometer 40, wiper arm 42 and thenback to the input terminals 28. The second path is from the inputterminal 26 through the wiper arm 34, the right-hand portion ofpotentiometer 32 and then through the thermistor 38, another relay coil52, and the right-hand portion of potentiometer 40 and its wiper arm 42back to the input terminal 28.

Thermistor 38 is an example of a sensing means which may be used tosense the amount of liquid carried in a gas stream, i.e. the quality, ofthe suction gas at the input of the compressor 10. The termistor is ofthe self-heating type and is connected through an isolating transformer56 to a pair of power input terminals 58 and 60. The transformerprovides power to heat the thermistor but is electrically isolated frombridge circuit 24 so that the thermistor heating current I and thebridge circuit current I; are independent of each other.

The thermistor has a negative temperature coefficient, that is, itsresistance decreases as its temperature increases. The temperature ofthe thermistor changes as the quality of the suction gas changes. Thistemperature change extends, for example, from approximately 35F when therefrigerant is 100 percent liquid at 35F evaporator temperature toapproximately 250F when the refrigerant is 100 percent gas at 35Fevaporator temperature. The thermistor is normally heated to atemperature very much higher than the temperature of the suction gas.The greater the amount of liquid in the suction gas, the greater thecooling efiect on the thermistor due to vaporization of the liquid as itflows over the thermistor. Therefore, the thermistor is at its high esttemperature when the refrigerant is 100 percent gas. Because of thislarge temperature change of the thermistor due to changes in quality ofthe refrigerant, the thennistor sensor is relatively unaffected by therelatively small normal temperature variations of the suction gas.

Relay coils 50 and 52 are magnetically linked to a movable armature 64which is normally floating in a neutral or centered position, asillustrated; that is, when bridge 22 is balanced, the current throughneither of the coils 50 and 52 is sufficient to produce a magnetic fieldstrong enough to attract armature 64 which then T remains in theposition illustrated in FIG. 2. A fixed relay contact 70 is connectedthrough a power relay coil 72 to a power input terminal 74. Anotherfixed relay contact 76 is connected through another power relay coil 78to the input terminal 74. Movable switch contacts 80 and 82 areconnected via a conductor 84 to the other power input terminal 86. Whenrelay coil 72 is energized, it closes a normally open switch 88 which isconnected in series with the forward drive winding 90 of the reversiblemotor 46. In like manner, when relay coil 78 is energized, it closes anormally open switch 91 which is connected in series with the reversedrive winding 92 of motor 46. Power is applied to terminals 94 and 96and then'to the motor drive winding corresponding to the closed one ofthe switches.

In operation, the calibration rheostat wiper arm 30 is adjusted tobalance the bridge circuit 24 for normal operating conditions. As anexample, the rheostat is adjusted so that the bridge is balanced whenthe quality of the compressor suction gas as sensed by thermistor 38 is95 percent, i.e. the gas contains 5 percent of droplets of liquidrefrigerant.

When the quality of the suction gas increases to 97 percent, forexample, the temperature of the thermistor in the suction gas in thecompressor inlet pipe 16 increases and the resistance of thermistor 38decreases, thereby unbalancing the bridge circuit 24 so that morecurrent flows through coil 52 and less current flows through coil 50.When the current through relay 52 reaches a predetermined valuedetermined by the design characteristics of the relay, armature 64 isattracted to the right, thereby closing contacts 76 and 82 andenergizing power relay coil 78 which closes switch 91. Consequently, thereverse winding 92 of motor 46 is energized to open valve 20, andthereby increase the flow rate of liquid refrigerant from the condenser12 to the evaporator 14.

Motor 46 also moves the wiper arm 42 to the left a distanceproporational to the change of position of valve 20 to rebalance thebridge circuit, and thereby maintain valve 20 in its new position untilthe bridge is again unbalanced by a change in the quality of the suctiongas. When the bridge is rebalanced, the current through relay coil 52drops to a value insufficient to keep contacts 76 and 82 closed, andarmature 64 will return to the central position as illustrated, therebydeenergizing relay 78 and opening switch 91.

In like manner, if the quality of the compressor suction gas drops, forexample to 93 percent, the temperature of the thermistor 38 decreasesand its resistance increases, thereby increasing the current flowthrough relay coil 50 and decreasing the current flow through relay coil52. Consequently, armature 64 will be attracted to the left to closecontacts and 80, thereby energizing power relay coil 72 to close switch88 and energize the forward drive winding 90 in the reversible motor 46.This will cause motor 46 to change the position of valve 20 so that theliquid refrigerant flow rate therethrough is decreased.

The bridge circuit 24 is again rebalanced because motor 46 moves thewiper arm 42 to the right to increase the resitance in the path throughrelay coil 50, thereby balancing the current flow through relay coils 50and 52 so that armature 64 again returns to its neutral or centralposition as illustrated. Therefore, contacts 70 and are opened, relaycoil 70 is deenergized, switch 88 is opened, and motor drive winding isde-energized, so that valve 20 remains in its new position'until thebridge circuit is again unbalanced by a change in the quality of thesuction gas.

The balance of the bridge 24 is also determined by the position of thewiper arm 34 on the potentiometer 32. Wiper arm 34 is connected viamechanical linkage 36 to the compressor capacity control slide valve 22in the compressor 10. If the position of the slide valve 22 changes dueto a change in compressor capacity, the position of the wiper arm 34 iscorrespondingly changed, thereby unbalancing the electrical currentflowing through the relay coils 50 and 52. In a manner similar to thatalready described, when the unbalancing becomes great enough, armature64 is atfiacted to close either contacts 70, 80 or 76, 82 to energizeone of the motor drive windings 90 and 92 of the reversible motor 46,which then changes the position of the liquid refrigerant flow controlvalve 20 to compensate for the change in compressor capacity asreflected by the change in position of the slider valve 22. Again, aspreviously described, the bridge is rebalanced by means of mechanicallinkage 44 connected between the motor 46 and the wiper arm 42 of thebridge potentiometer 40.

More specifcally, when the compressor capacity increases, slide valve 22moves wiper arm 34 to the right, thereby causing contacts 76 and 82 toclose so that refrigerant flow control valve 20 is moved to a more fullyopen position to increase the flow of liquid refrigerant from condenser12 to evaporator 14. Consequently, the compressor mass flow rate and theliquid refrigerant mass flow rate through valve 20 are kept essentiallythe same at all times without any time lag.

The signal introduced into bridge circuit 24 by thermistor 38 makesappropriate fine adjustments in the position of the flow control valve20 to keep the suction gas quality at a desired optimum value, e.g. 95percent, regardless of changes in the system refrigerant charge,operating pressures, oil foaming in the evaporator, and the like.

In an alternative embodiment, potentiometer 32 and its wiper arm 34 arereplaced by a dummy resistor so that only thermistor 38 provides thecontrol signal to bridge 24 for controlling the position of flow controlvalve 24 In this case, valve 20 is not controlled by the capacitysetting of the slide valve 22 of compressor 10, but rather is controlledsolely by the quality of the suction gas. This control is important inoil-injected screw compressors, because too low a suction gas qualityaffects the efficiency of the oil separator.

While the invention has been particularly shown and described withreference to the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined in the appended claims. More specifically, eventhrough the bridge circuit 24 has been described as an electromechanicalcircuit, an equivalent transistorized circuit is within the scope of theinvention.

I claim:

1. In a refrigeration system including a compressor, a condenser and anevaporator connected in series in a closed loop wherein the compressoris a load controlled variable capacity compressor and the rate of flowof liquid refrigerant to the evaporator is conu'olled in accordance withthe capacity of the compressor to maintain the liquid refrigeraant flowrate through the evaporator substantially equal to the mass flow ratethrough the compressor, the improvement comprising:

a. valve means for controlling the rate of flow of liquid refrigerantinto the evaporator;

b. quality sensing means for producing a first control signal indicativeof the quality of the refrigerant suction gas at the input of thecompressor;

0. circuit means responsive to said first control signal for controllingthe position of said valve means to control said rate of flow of liquidrefrigerant so that the quality of the suction gas is maintained at adesired optimum valve;

d. compressor capacity sensing means for producing a second controlsignal indicative of the capacity of the compressor; and

e. said circuit means being additionally responsive to said secondcontrol signal to control the position of said valve means in accordancewith both said first and second control signals so that said rate offlow of liquid refrigerant is controlled by both the capacity of thecompressor and also the quality of the refrigerant suction gas at theinput of the compressor.

2. A liquid refrigerant feed control as defined in claim 1 wherein saidcircuit means comprises:

a. a normally balanced bridge circuit having first and second circuitpaths normally carrying substantially equal currents;

b. means connecting said quality sensing means in said first circuitpath; 2

0. means connecting said capacity sensing means in said second circuitpath;

(1. first relay means connected in said first circuit path;

e. second relay means connected in said second circuit path; and

f. valve position control means connected to said valve means andselectively responsive to the operation of said first and second relaymeans for changing the position of said valve means, whereby changes insaid first and second control signals unbalance said bridge circuit tocause unequal currents to flow in said first and second circuit paths,thereby operating one of said relay means to operate said valve positioncontrol means which changes the position of said valve means so thatsaid rate of flow of the liquid refrigerant is determined by both ofsaid first and second control signals.

1. In a refrigeration system including a compressor, a condenser and an evaporator connected in series in a closed loop wherein the compressor is a load controlled variable capacity compressor and the rate of flow of liquid refrigerant to the evaporator is controlled in accordance with the capacity of the compressor to maintain the liquid refrigeraant flow rate through the evaporator substantially equal to the mass flow rate through the compressor, the improvement comprising: a. valve means for controlling the rate of flow of liquid refrigerant into the evaporator; b. quality sensing means for producing a first control signal indicative of the quality of the refrigerant suction gas at the input of the compressor; c. circuit means responsive to said first control signal for controlling the position of said valve means to control said rate of flow of liquid refrigerant so that the quality of the suction gas is maintained at a desired optimum valve; d. compressor capacity sensing means for producing a second control signal indicative of the capacity of the compressor; and e. said circuit means being additionally responsive to said second control signal to control the position of said valve means in accordance with both said first and second control signals so that said rate of flow of liquid refrigerant is controlled by both the capacity of the compressor and also the quality of the refrigerant suction gas at the input of the compressor.
 2. A liquid refrigerant feed control as defined in claim 1 wherein said circuit means comprises: a. a normally balanced bridge circuit having first and second circuit paths normally carrying substantially equal currents; b. means connecting said quality sensing means in said first circuit path; c. means connecting said capacity sensing means in said second circuit path; d. first relay means connected in said first circuit path; e. second relay means connected in said second circuit path; and f. valve position control means connected to said valve means and selectively responsive to the operation of said first and second relay means for changing the position of said valve means, whereby changes in said first and second control signals unbalance said bridge circuit to cause unequal currents to flow in said first and second circuit Paths, thereby operating one of said relay means to operate said valve position control means which changes the position of said valve means so that said rate of flow of the liquid refrigerant is determined by both of said first and second control signals. 